1. General Introduction ────────────────────────────────────────────────────────────────
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1 ──────────────────────────────────────────────────────────────── 1. General Introduction ──────────────────────────────────────────────────────────────── 1.1 Community texture The idea of classifying plants according to their morphology has been traced to Theophrastus (371-286 BC) who recognised four growth forms among terrestrial plants (Barkman 1988). In the modern context, von Humboldt (1806) has been credited with developing the earliest classification of plant physiognomy. Following Darwin, there was much interest in identifying plant features of adaptive significance. Warming (1909) and Schimper (1903) were among the early workers who postulated an adaptive significance for morphology, and for other classes of species characters such as phenology, physiology and life-history. Raunkiaer's (1934) life-form system, still widely used today (e.g. Danin & Orshan 1990; Floret et al. 1990; Shmida & Werger 1992), was based on the position of the persistent apical meristem relative to the soil surface, which Raunkiaer considered of prime adaptive importance. Numerous schemes for classifying plants according to different aspects of their morphology have been developed since (e.g. Luther 1949; Danserau 1951, 1957; Hallé, Oldeman & Tomlinson 1978; Halloy 1990). Although some workers have been cautious in attributing general adaptive significance to species characters (e.g. Du Rietz 1931), most of these studies have been motivated by an assumption that the characters are meaningful functionally, that is, that they influence a species' ability to establish, grow, and reproduce in a given environment. If the assumption of the functional significance of species characters is valid, classifications of these characters should provide a valuable basis for characterising vegetation, both for comparison among sites, environments or regions, and for prediction. Although a number of studies have been concerned primarily with the relationships between functional characteristics of vegetation and the environment (e.g. Whitehead 1954; Parsons & Moldenke 1975; Werger & Ellenbroek 1978; Bongers & Popma 1990; Smith et al. 1995 [see Appendix C]), vegetation has more generally been characterised by the taxonomic affinities of the species present, supplemented perhaps by the physiognomy of the dominant species (e.g. Braun-Blanquet 1932; Ellenberg 1963; Rieley & Page 1990). Knowledge of the taxonomic composition of vegetation can provide only indirect evidence for the functional adaptations of species present, to the extent that certain species, genera and to a lesser extent higher taxonomic groups may have known ecological affinities (Whittaker 1975). Comparisons of vegetation among sites and studies is constrained by a progressive reduction in floristic overlap from the local to the regional and subsequently global scale, if assemblages are characterised only by their taxonomic composition. With the aim of promoting a less species-oriented approach to the classification of 2 vegetation, Barkman (1979) proposed a scheme for classifying plant communities according to texture and structure. Texture was defined as the `qualitative and quantitative composition of vegetation as to different morphological elements', while `structure' referred to the spatial configuration of these elements. This terminology was attributed to Doing (unpublished). Though the classification scheme proposed was based solely on plant morphology, Barkman (op. cit.) recognised that texture could also include other species characters, and even interactions between species. Further, there is no reason why the term could not be extended to apply to assemblages of organisms other than plants. In the present study `texture', or `community texture', is used to refer to assemblage-wide spectra of characters of known or potential functional importance (an assemblage is defined as any group of co-occurring species, i.e. a community or guild, as defined below). Possible examples of texture would include the frequency distribution of leaves of different sizes in the vascular plant guild of a particular community, or the spectrum of body weights of insectivorous birds encountered in a sampling area. 1.2 Functional groups and guilds The terms functional group (Cummins 1973), functional type (Gitay & Noble, in press), adaptive syndrome (Root & Chaplin 1976), clique (Yodzis 1982) and guild (Schimper 1903) all refer to groups of functionally-related species. Each term has its own definition, and most have been ascribed more than one meaning in different studies. Several reviews have attempted to bring order to the semantic chaos (Hawkins & MacMahon 1989; Simberloff & Dayan 1991; Wilson, in prep.), with the result that even more definitions abound. The closely-related concepts of functional groups and guilds are most commonly invoked in the literature, and are discussed further here. FUNCTIONAL GROUPS The functional groups conceived by Cummins (1973) were of invertebrate species utilising the same types of food. Later, functional groups were defined for other types of organism, the basis of classification being either the resources used (e.g. MacMahon et al. 1981), or both the resources and the way they are used (e.g. Cummins & Merritt 1984), a usage synonymous with guilds sensu Root (1967). Plant functional groups have generally been defined according to morphology or life history characteristics rather than resource use (e.g. Grime 1977; Boutin & Keddy 1993; Golluscio & Sala 1993), probably because differences in the resources used are not so obvious for plants as for animals, which have distinct feeding niches (Simberloff & Dayan 1991). Recently Körner (1994) has advocated extension of the functional groups concept to encompass levels of biological organisation other than species, defining them as `elements that 3 bear a certain set of common structural and/or process features,' where the elements concerned could be, apart from species populations, higher-order biological entities such as communities or ecosystems (sic); or lower-order entities: individuals, organs, tissues, cells etc. Functional groups have been advocated as a means of simplifying ecosystem models (Botkin 1975; Woodward 1987), or as a basis for predicting vegetation changes in response to perturbation (Boutin & Keddy 1993) or climate change (Prentice et al. 1992). GUILDS The original guilds of Schimper (1903) were four types of plants depending on other plants in a different way: lianes, epiphytes, saprophytes and parasites. Clements (1905) used the same term to refer to groups of species migrating together. Most frequently cited, however, is the definition of Root (1967), who used the term `guild' to refer to groups of species `using a similar class of resources in a similar way.' In this context, guilds are groups of species that possess similar alpha (habitat) niches (Pickett & Bazzaz 1978). Guilds may thus represent the `basic building blocks' of communities (Hawkins & MacMahon 1989), major ecological groups that have been `molded by adaptation to the same class of resources' (Root 1967). Guild structure might be repeated in communities in similar environments, even if the species composition is not (Hawkins & MacMahon 1989; Wilson 1989). Pianka (1980) considered that guild associates (i.e. members of the same guild), making demands on the same resources, would interact with each other more than with species outside the guild, indeed, that guilds might be `arenas of intense ... competition.' Wilson (in prep.) argues for a distinction between alpha guilds (of the kind envisaged by Root [1967]), and beta guilds, comprising species with a similar beta niche, i.e. geographic or environmental distribution (Pickett & Bazzaz 1978). This distinction would be valuable, since concepts of competitive exclusion, niche differentiation, species packing etc. apply solely to species overlapping in their alpha niche, and so competing for some of the same resources. There can be no competition for environmental conditions (the properties that define beta niches). Therefore beta guilds, comprising species that have similar beta niches, would lack many of the characteristics ascribed to guilds sensu stricto above. As is the case with functional groups, plant guilds are rarely defined directly according to resource use, but rather in terms of species attributes that might be expected to show some correlation with niche (e.g. Cornelius et al. 1991). An exception is the practice of treating vertical strata or sinusiae in plant communities as guilds (e.g. Wilson 1989; Wilson et al. 1995), since some resources, notably light, would be partitioned among strata as envisaged by Root (1967) for guilds. Phylogeny has often been used implicitly or explicitly as a criterion in defining guilds (Schoener 1986; Jaksi_ 1981). MacNally & Doolan (1986) have even suggested inclusion of `closely related' in the definition of guilds. The inclusion of taxonomy as a criterion for guild 4 membership is consistent with Root's (1967) definition to the extent that species within higher taxonomic groups are often functionally, as well as phylogenetically, distinct from species belonging to other taxa of the same rank (Whittaker 1975). Thus guilds such as `vascular plants' or `bryophytes' may fall within Root's (1967) concept. Assignment of species to guilds or functional groups has generally been done either qualitatively on the basis of some known or inferred feature of species' ecology (e.g. food type: Root [1967], sinusia: Wilson 1989), ecology plus phylogeny